1. Field of the Invention
The present invention relates generally to continuously variable transmissions and more specifically to hydraulic control thereof.
2. Background Art
Continuously variable transmissions comprise, inter. alia an input shaft rotatable by a prime mover, an output shaft connected to vehicle wheels and a ratio varying component (hereinafter referred to as a “variator”) disposed between the input and output shafts. The variator is typically controlled by means of hydraulic pressures which can adjust the effective ratio of the variator in accordance with a driver's demands, road conditions and the like. Under steady or smoothly-changing conditions the required flows of hydraulic fluid are relatively small. However, problems can sometimes occur during rapid ratio changes of the variator, for example during braking, rapid engine acceleration or abuse conditions. During such rapid ratio changes, the flows of fluid can be excessive, resulting in adverse system controllability.
This is particularly, but not exclusively, applicable in the case of torque control variators.
Major components of a known torque-control variator 10 of the “full toroidal”, toroidal-race rolling traction type are illustrated in FIG. 1. Here, two input discs 12, 14 are mounted upon a drive shaft 16 for rotation therewith and have respective part toroidal surfaces 18, 20 facing toward corresponding part toroidal surfaces 22, 24 formed upon a central output disc 26. The output disc is journalled such as to be rotatable independently of the shaft 16. Drive from an engine or other prime mover, input via the shaft 16 and input discs 12, 14, is transferred to the output disc 26 via a set of rollers disposed in the toroidal cavities. A single representative roller 28 is illustrated but typically three such rollers are provided in both cavities. An end load applied across the input discs 12, 14 by a hydraulic end loading device 15 provides contact forces between rollers and discs to enable the transfer of drive. Drive is taken from the output disc to further parts of the transmission, typically an epicyclic mixer, as is well known in the art and described eg. in European patent application 85308344.2 (published as EP 0185463). Each roller is journalled in a respective carriage 30 which is itself coupled to a hydraulic actuator 32 whereby a controlled translational force can be applied to the roller/carriage combination. As well as being capable of translational motion the roller/carriage combination is able to rotate about an axis determined by the hydraulic actuator 32 to change the “tilt angle” of the roller and so move the contacts between rollers and discs, thereby varying the variator transmission ratio, as is well known to those skilled in the art.
As mentioned above, the illustrated variator is of the type known in the art as “torque control”. The hydraulic actuator 32 exerts a controlled force on the roller/carriage and for equilibrium this must be balanced by the reaction force upon the roller resulting from the torques transmitted between the disc surfaces 18, 20, 22, 24 and the roller 28. As is well known in the art, the centre of the roller is constrained to follow the centre circle of the torus defined by the relevant pair of discs. The axis determined by the actuator 32 is angled to the plane of this centre circle. This angle is referred to as the “castor angle”. The well known result of this arrangement is that in use each roller automatically moves and precesses to the location and tilt angle required to transmit a torque determined by the biasing force from the actuator 32.
The biasing force is controlled by means of a hydraulic circuit through which fluid is supplied to the actuators at variable pressure.
It will be appreciated that while the equilibrium position of the rollers is determined by the balance of the reaction force and the applied biasing force, there is the potential for unwanted oscillatory motion of the roller/carriage combination about this position, with resulting impairment of transmission function. More than one mode of oscillation is possible but in the simplest such mode all rollers oscillate in unison and this oscillatory motion is accompanied by a corresponding flow of fluid in the hydraulic circuit.
Damping of such oscillation can be provided by means of the hydraulic circuit and specifically by restricting or throttling fluid flow to and from the actuators 32. During a change in variator transmission ratio, the rollers 28 must move and precess to new positions, fluid thus being expelled from one side of the pistons of the actuators 32 and taken in on the other side. Under these conditions, if fluid flow in the hydraulic circuit is suitably restricted, pressure is increased in the hydraulic circuit on the side of fluid expulsion and diminished on the other side of the circuit, modifying the net force exerted on the rollers by the actuators such as to tend to resist roller motion and thus to create a torque between the variator input and output discs.
The effect is two-fold:                i. damping is provided, which helps to deliver smooth non-oscillatory variator response, particularly when installed in a mechanical power train; but        ii. the torque created resists required ratio change, which can impair transmission performance during rapid transient events such as rapid braking and rapid acceleration.        
Particularly stringent requirements are imposed on the transmission by such “transients”—rapid changes in the operating conditions of the vehicle requiring correspondingly rapid changes of transmission ratio. An emergency stop or “brake to rest” is one example. In order to maintain engine speed and to avoid stalling the engine during a brake to rest, rapid ratio change is required of the variator. This is particularly significant in a transmission of the “geared neutral” type in which the variator remains coupled to the vehicle's wheels even while the wheels are stationary—that is, in vehicles lacking a clutch or other means to isolate wheels and engine. The high rate of ratio change required during rapid brake to rest corresponds to a rapid motion of the variator rollers and their associated pistons. Large flows are created in the hydraulic control circuit. If adequate hydraulic flow to accommodate such motion is not available—particularly because such flow is restricted—the rollers can fail to move with sufficient speed, leading eg. to an engine stall. Within the hydraulic circuit the effect can be a large increase in pressure on one side of the circuit and a large fall in pressure on the other side of the circuit. The result must be a large net biasing force on the roller/carriage combinations and this is reflected in a large variator torque which is the cause of the engine stall.